The invention relates to the general field of semiconductor integrated circuits with particular reference to connection metallurgy.
The flip chip approach for making chip I/O connections to the next packaging level has received widespread application. At the heart of this technology are individual solder pellets or bumps, each of which contacts a single chip I/O pad and extends upwards away from the chip surface. A common technique for forming solder bumps is to form a mold of photoresist and to then fill the mold with solder, either through electroplating or by application of a solder paste. This is illustrated in FIG. 1 where semiconductor wafer 11, containing completed integrated circuits, is seen to have a top layer 12 in which have been formed recesses for locating the solder bumps. Conductive vias that make contact to the underlying chip and pass through layer 12 are not explicitly shown.
Layer 13 is a layer of conductive material(s) that initially cover(s) the entire chip thereby enabling photoresist mold 14 to be filled with solder 15 that is grown therein through electroplating or as a paste. If the electroplating method is used, a layer of UBM (under ball metallurgy) made up of materials such as titanium, copper, or nickel is first grown over the surface of layer 13 (UBM not shown as a separate layer).
As an alternative to electroplating, a pad of UBM material may be formed first on the upper surface of layer 12 followed by the placement of photoresist mold 14 in alignment therewith, the mold being then filled with a solder paste of lead-tin, silver-tin, or silver-copper-tin, etc. through printing or plating. Whichever process is used, the net result after the photoresist mold has been removed, is solder bump 15 that sits on pad 12. The latter may be a single UBM layer or a UBM layer over a seed layer. This is illustrated in FIG. 2.
Part of the standard process for forming solder bumps is a remelt step where the bumps are gently heated to ensure their adhesion to the underlying UBM pads. As a result of this, the bumps assume a spherical shape (due to surface tension forces) and each solder bump becomes a solder ball 35, as illustrated in FIG. 3. This means that for every increase in solder bump height there will be a corresponding increase in solder bump width. Because of this the mean spacing between solder bumps has, in the prior art, been limited to greater than about 150 microns.
The present invention teaches a method to overcome this limitation.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,130,149, Chien et al. discuss the bump process in their Background of the invention section. Seppala et al. disclose a single exposure DFR and bump process in U.S. Pat. No. 5,665,639. A two step bump process is described in U.S. Pat. No. 6,228,681 (Gilleo), but it is different from that of the present invention. U.S. Pat. No. 6,218,281 (Watanabe) and U.S. Pat. No. 6,077,765 (Naya) are related bump processes.
It has been an object of at least one embodiment of the present invention to provide a process for forming solder bumps for use in a flip chip.
Another object of at least one embodiment of the present invention has been that, after remelt, the resulting solder balls have an elongated shape whereby a high area density of bumps can be achieved without sacrificing solder ball height.
Still another object of at least one embodiment of the present invention has been that said process be fully compatible with current methods for forming solder bumps.
These objects have been achieved by preparing the mold in which the solder is formed in two steps. In a first exposure, photoresist is patterned to form a conventional cylindrical mold. However, exposure and development are adjusted in such a way that a layer of unexposed and undeveloped resist remains covering the floor of the mold. This residual resist layer is given a second exposure and, after development, forms an annular insert in the bottom of the first mold. After the mold has been filled with solder (either through electroplating or by using solder paste) it is removed, the result being a solder bump made up of two contiguous coaxial cylinders the upper one having the larger diameter. After remelt, bumps having this shape form oblate spheroids, rather than spheres, so that greater ball height is achieved without the need to sacrifice areal density.